A tire is a complex body of fiber/steel/rubber, and generally has a structure as illustrated in FIG. 1. Herein, body ply is, also called carcass, a reinforcing cord layer inside the tire, and supports the entire load of vehicle, maintains the tire shape, and withstands against a shock, and is required to have high fatigue resistance against bending and stretching movement during driving.
A polyester synthetic fiber such as polynaphthalene terephthalate is generally applied to the body ply, that is, tire-cord.
The synthetic fiber cord has high tenacity to greatly contribute to the durability improvement of tire. However, it has a disadvantage of reducing elasticity and dimensional stability of the tire after curing process due to its high heat shrinkage ratio. In order to make up for this disadvantage, many studies have been made to improve dimensional stability of the cord through an additional process such as PCI (Post Cure Inflation). In particular, high tenacity fibers for industrial applications are able to show high strength by increasing a drawing ratio at a low speed. However, they still have high heat shrinkage ratio and low elasticity, and thus the PCI process is required.
Subsequently, an ultra high-speed spinning technique was employed in the manufacturing process of tire-cord, and thus it is possible to manufacture a polyester tire-cord having high modulus low shrinkage (HMLS) properties without the PCI process.
In order to manufacture the tire-cord having high modulus low shrinkage (HMLS) properties, an undrawn fiber having high crystallinity should be used. Since the undrawn fiber having high crystallinity has a relatively narrow region to be drawn, non-uniform drawing or breakage due to friction easily occurs when the undrawn fiber is drawn at a ultra-high speed and a high drawing ratio using a ultra high-speed spinning equipment.
For this reason, there is a limitation in the drawing ratio of the undrawn fiber having high crystallinity when applied to the ultra high-speed spinning equipment, and sufficient drawing cannot be given to the fiber, resulting in a great reduction in tensile strength of the drawn fiber. In particular, it is more difficult to secure a sufficient distance between holes in the spinneret and uniform cooling during the manufacturing process of the drawn fiber having a high fineness of 2000 denier or more and the tire-cord, and thus a great reduction in physical properties such as tenacity occurs, and a tire-cord having uniform physical properties cannot be obtained.
In more detail, when the drawn fiber having a high fineness and the tire-cord are manufactured using the known spinning equipment, the amount of polymers staying in a spinning chimney is increased to generate non-uniform cooling between inner and outer layers, and thus it is difficult to produce a drawn fiber having uniform physical properties and monofilaments with an uniform cross-sectional area and a tire-cord, and the fineness of monofilaments is increased to increase a discharge speed of the melt in the spinneret, and thus it is difficult to provide sufficient spinning draft. Thus, an orientation difference occurs due to the larger cooling difference between inner and outer layers of monofilaments so as to reduce tenacity, and dimensional stability is also reduced due to the low spinning draft, resulting in unsatisfactory characteristics of tire-cord.
In order to solve the problems, it was previously considered or applied that an undrawn fiber having low fineness was produced using the ultra high-speed spinning technique, followed by twisting during the drawing process. However, this twisting method requires high production costs, and tenacity is damaged by friction due to twisting, and thus there are many difficulties in productivity improvement in manufacturing of a fiber with a thick fineness and sufficient tenacity.
Recently, as the use of RADIAL tire is increased because of the above problems, it is required to provide a tire-cord having a large fineness and excellent and uniform physical properties, but satisfactory progress has not been achieved yet. Thus, there is a need to develop a technique for effectively manufacturing a poly(ethylene terephthalate) drawn fiber having superior strength, dimensional stability and uniform physical properties and having a high fineness of 2000 denier or more without breakage or deterioration in physical properties during the manufacturing process, and a tire-cord.
Furthermore, poly(ethylene terephthalate) having a high I.V. has been generally used as a method for increasing the tenacity of the poly(ethylene terephthalate) drawn fibers. When the intrinsic viscosity of the polymer is raised, the spinning tension increases, and the orientation of the undrawn fiber and the formation of tie-chains connecting crystals increase. Thus, the manufactured drawn fiber can show superior tenacity, and a tire-cord manufactured using the drawn fiber can also show superior tenacity. However, as the intrinsic viscosity of the polymer is increased, a large difference in intrinsic viscosities between the inner and outer layers of the polymer chip is generated. Therefore, the spinnability deteriorates due to the heterogeneity of the viscosity, and the increased intrinsic viscosity of the polymer increases the melt viscosity of the polymer, which increases discharge pressure in the spinneret during spinning, leading to deterioration in spinnability and productivity. In order to solve these problems, a method of reducing the melt viscosity by increasing spinning temperature has been used, but reduction of polymerization degree is generated due to thermal degradation and hydrolysis of the polymer, and thus it is difficult to achieve high tenacity. To solve this problem, addition of various lubricants or viscosity-reducing agents has been also suggested. An example of the former is stearic acid, but its addition to a resin has a disadvantage of reducing the melt viscosity and molecular weight at the same time. An example of the latter is a polycarbonate-based amide compound, but at least several % of viscosity-reducing agent should be added in order to obtain a fiber having high tenacity through sufficient effect of the viscosity-reducing agent. Thus, it is economically unfavorable, and the presence of residues is increased by partial agglomeration and poor dispersion of the viscosity-reducing agent, resulting in deterioration of tensile property and processability.